Low-temperature degradation increases the cyclic fatigue resistance of 3Y-TZP in bending.

OBJECTIVE: Due to past failures of orthopedic 3Y-TZP femoral implants linked to accelerated tetragonal-to-monoclinic phase transformation (t → m), the susceptibility to 'low-temperature degradation' or 'ageing' of 3Y-TZP has been advertised as detrimental to its long-term structural stability. However, no systematic mechanistic experiments on the fatigue resistance of aged 3Y-TZP under cyclic loading can support such statement. In this study, we aim to clarify this issue. METHODS: Here we evaluate the subcritical crack growth behavior of 3Y-TZP under cyclic loading after 0-50 h of accelerated ageing in an autoclave at 134 °C. The same 3Y-TZP sintered at two different temperatures (1450 °C or 1650 °C) allows for the comparison of materials containing grains with different susceptibilities to transformation. The volume fraction of surface transformed grains was measured using Raman spectroscopy, and the depth of the transformed surface layer from trenches milled with a Focus-Ion Beam. The fracture toughness before and after ageing was determined using the Chevron-notch Beam method. The quasi-static flexural strength was measured in dry conditions and the cyclic lifetime in water at 10 Hz and R-ratio = 0.3 in 4-point-bending at different applied stresses. The fatigue parameter n was derived from 3 different methods, namely SN curves, crack velocity plots and Weibull distributions. RESULTS: The progression of transformation showed linear kinetics with higher rates for the 3Y-TZP sintered at 1650 °C. Accelerated transformation induced severe crack formation within the transformed layer with parallel orientation to the surface plane, which supposedly behaved as the critical crack size population governing fracture. The stress intensity factor within the transformed layer was increased due to compressive stresses. Consequently, the fatigue parameter n increased consistently from 5 to 50 h of ageing, regardless of the derivation method, suggesting an increased resistance against crack growth during cyclic loading in bending. SIGNIFICANCE: Our results do not support the long suggested negative clinical implications of LTD regarding mechanical performance, to the contrary, LTD seems to increase the resistance against subcritical crack growth in a humid environment in bending.

Strength and fracture toughness of zirconia dental ceramics.

OBJECTIVE: The aim of the paper is to determine and discuss the correlation between the fracture toughness and the fracture stress in zirconia transforming ceramics with a small artificial crack. As an R-curve behaviour is usually present in transforming ceramics for both small and long cracks, predictions of the fracture stress can only be done with an accurate knowledge of the R-curve and crack dimensions. METHODS: First, basic concepts of fracture mechanics, strength and testing of ceramic materials are introduced. This is followed by a very brief introduction to zirconia dental ceramics and to strength degradation by hydrothermal ageing of 3Y-TZP. Fracture toughness of 3Y-TZP and 12Ce-TZP are then determined for a short (∼50µm) sharp edge crack produced by ultra short pulsed laser ablation on prismatic bending bars in four point bending. The crack size is small but large enough for controlling fracture and for applying elastic fracture mechanics. RESULTS: In both materials the determined fracture toughness is similar, in spite of their difference R-curves. The results of fracture toughness and fracture stress are analysed by using a simple function to represent the R-curve, but which contains the main ingredients of experimental R-curves extracted from the literature either for short or long cracks in 12Ce-TZP. SIGNIFICANCE: It is concluded that the high R-curves reported in the literature for long and short cracks in 12Ce-TZP and 3Y-TZP might have only a marginal influence on the fracture resistance with cracks of the size studied. This effect is of more significance in 12Ce-TZP. The use of an ideal and simple model of R-curve is presented as a useful guide to predict whether the fracture stress will be enhanced by an existent R-curve.

A parametric study of laser interference surface patterning of dental zirconia: Effects of laser parameters on topography and surface quality.

OBJECTIVE: The aim of this work is to generate micrometric linear patterns with different topography on dental grade zirconia by means of UV laser interference and to assess the quality of the produced surface, both in term of the geometry produced and of the surface damage induced in the material. METHODS: The third harmonic of a Q-switched Nd:YAG laser (355nm, pulse duration of 10ns and repetition rate of 1Hz) was employed to pattern the surface of 3Y-TZP with micrometric-spaced lines. The resulting topography was characterized with White Light Interferometry and Scanning electron microscopy: pattern depth (H), amplitude roughness parameters (Sa, filtered-Sa), Fourier spatial analysis and collateral damages were related to laser fluence and number of pulses employed. RESULTS: With our experimental setup, line-patterning of zirconia surfaces can be achieved with periodicities comprised within 5 and 15µm. Tuning laser parameters allows varying independently pattern depth, overall roughness and surface finish. Increasing both fluence and number of pulses allows producing deeper patterns (maximum achievable depth of 1µm). However, increasing the number of pulses has a detrimental effect on the quality of the produced lines. Surface damage (intergranular cracking, open porosity and nano-droplets formation) can be generated, depending on laser parameters. SIGNIFICANCE: This work provides a parametric analysis of surface patterning by laser interference on 3Y-TZP. Best conditions in terms of quality of the produced pattern and minimum material damage are obtained for low number of pulses with high laser fluence. With the employed method we can produce zirconia materials with controlled topography that are expected to enhance biological response and mechanical performance of dental components.

Selective etching of injection molded zirconia-toughened alumina: Towards osseointegrated and antibacterial ceramic implants.

Due to their outstanding mechanical properties and excellent biocompatibility, zirconia-toughened alumina (ZTA) ceramics have become the gold standard in orthopedics for the fabrication of ceramic bearing components over the last decade. However, ZTA is bioinert, which hampers its implantation in direct contact with bone. Furthermore, periprosthetic joint infections are now the leading cause of failure for joint arthroplasty prostheses. To address both issues, an improved surface design is required: a controlled micro- and nano-roughness can promote osseointegration and limit bacterial adhesion whereas surface porosity allows loading and delivery of antibacterial compounds. In this work, we developed an integrated strategy aiming to provide both osseointegrative and antibacterial properties to ZTA surfaces. The micro-topography was controlled by injection molding. Meanwhile a novel process involving the selective dissolution of zirconia (selective etching) was used to produce nano-roughness and interconnected nanoporosity. Potential utilization of the porosity for loading and delivery of antibiotic molecules was demonstrated, and the impact of selective etching on mechanical properties and hydrothermal stability was shown to be limited. The combination of injection molding and selective etching thus appears promising for fabricating a new generation of ZTA components implantable in direct contact with bone. STATEMENT OF SIGNIFICANCE: Zirconia-toughened alumina (ZTA) is the current gold standard for the fabrication of orthopedic ceramic components. In the present work, we propose an innovative strategy to provide both osseointegrative and antibacterial properties to ZTA surfaces: we demonstrate that injection molding allows a flexible design of surface micro-topography and can be combined with selective etching, a novel process that induces nano-roughness and surface interconnected porosity without the need for coating, avoiding reliability issues. These surface modifications have the potential to improve osseointegration. Furthermore, our results show that the porosity can be used for drug delivery and suggest that the etched surface could reduce bacterial adhesion.

Roughness gradients on zirconia for rapid screening of cell-surface interactions: Fabrication, characterization and application.

Roughness is one of the key parameters for successful osseointegration of dental implants. The understanding of how roughness affects cell response is thus crucial to improve implant performance. Surface gradients, which allow rapid and systematic investigations of cell-surface interactions, have the potential to facilitate this task. In this study, a novel method aiming to produce roughness gradients at the surface of zirconia using hydrofluoric acid etching was implemented. The topography was exhaustively characterized at the microscale and nanoscale by white light interferometry and atomic force microscopy, including the analysis of amplitude, spatial, hybrid, functional, and fractal parameters. A rapid screening of the influence of roughness on human mesenchymal stem cell morphology was conducted and potential correlations between roughness parameters and cell morphology were investigated. The roughness gradient induced significant changes in cell area (p < 0.001), aspect ratio (p = 0.01), and solidity (p = 0.026). Nanoroughness parameters were linearly correlated to cell solidity (p < 0.005), while microroughness parameters appeared nonlinearly correlated to cell area, highlighting the importance of multiscale optimization of implant topography to induce the desired cell response. The gradient method proposed here drastically reduces the efforts and resources necessary to study cell-surface interactions and provides results directly transferable to industry. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 2502-2514, 2016.

Surface roughened zirconia: towards hydrothermal stability.

Surface roughness is needed in several yttria-stabilized zirconia components used in restorative dentistry for osseointegration or adhesion purposes. This can be achieved by different treatments, which may also modify the microstructure of the surface. Among them, sandblasting and chemical etching are widely used, but their effect on hydrothermal aging of zirconia is not fully understood. In the present work, the zirconia long-term stability of rough surfaces prepared by these techniques is analyzed and a method is proposed for preventing hydrothermal aging while maintaining the original surface appearance and mechanical properties. The method involves pressure infiltration of a Cerium salt solution on the roughened surfaces followed by a thermal treatment. The solution, trapped by surface defects and small pores, is decomposed during thermal treatment into Cerium oxide, which is diffused at high temperature, obtaining Ce co-doping in the near-surface region. In addition, the microstructural changes induced in the near-surface by sandblasting or chemical etching are removed by the thermal treatment together with surface defects. No color modification was observed and the final roughness parameters were in the range of existing implants of proved good osseointegration. The aging resistance of Ce co-doped materials was strongly enhanced, showing the absence of aging after artificial degradation, increasing in this way the surface mechanical integrity. The proposed treatment is easily applicable to the current manufacturing procedures of zirconia dental posts, abutments, crowns and dentures, representing a solution to hydrothermal aging in these and other biomedical applications.

Enhanced reliability of yttria-stabilized zirconia for dental applications.

An increasing number of dental applications based on yttria-stabilized zirconia (3 Y-TZP) have been developed in recent years as a result of the advances and versatility of dry-processing and soft machining at the pre-sintered state. Nonetheless, the long-term surface stability of these materials in humid environment is still a matter of concern and may limit its application. In this work, a simple method to prevent hydrothermal degradation on the zirconia surface is studied in detail. This method involves the infiltration of pre-sintered parts with optimized solutions containing Ce salts, leaving unchanged the other processing steps, allowing the diffusion of Ce during conventional sintering. Several pre-sintering conditions, solution concentrations and sintering temperatures were studied and characterized, obtaining working parameters for the production of zirconia parts with mechanical properties similar to standard 3 Y-TZP and high resistance to hydrothermal aging. This optimal combination was obtained with the 1150 °C pre-sintering temperature, 50 wt.% solution and sintering at 1450 °C, leading to a superficial CeO2 content of about 3 mol.%.

Osseointegration improvement by plasma electrolytic oxidation of modified titanium alloys surfaces.

Titanium (Ti) is a material frequently used in orthopedic applications, due to its good mechanical properties and high corrosion resistance. However, formation of a non-adherent fibrous tissue between material and bone drastically could affect the osseointegration process and, therefore, the mechanical stability of the implant. Modifications of topography and configuration of the tissue/material interface is one of the mechanisms to improve that process by manipulating parameters such as morphology and roughness. There are different techniques that can be used to modify the titanium surface; plasma electrolytic oxidation (PEO) is one of those alternatives, which consists of obtaining porous anodic coatings by controlling parameters such as voltage, current, anodizing solution and time of the reaction. From all of the above factors, and based on previous studies that demonstrated that bone cells sense substrates features to grow new tissue, in this work commercially pure Ti (c.p Ti) and Ti6Al4V alloy samples were modified at their surface by PEO in different anodizing solutions composed of H2SO4 and H3PO4 mixtures. Treated surfaces were characterized and used as platforms to grow osteoblasts; subsequently, cell behavior parameters like adhesion, proliferation and differentiation were also studied. Although the results showed no significant differences in proliferation, differentiation and cell biological activity, overall results showed an important influence of topography of the modified surfaces compared with polished untreated surfaces. Finally, this study offers an alternative protocol to modify surfaces of Ti and their alloys in a controlled and reproducible way in which biocompatibility of the material is not compromised and osseointegration would be improved.

Effect of sandblasting and residual stress on strength of zirconia for restorative dentistry applications.

Sandblasting is a commonly used surface treatment technique for dental crowns to improve the adhesion of the mating parts of a restoration. The goal of this work is to study the effect of different sandblasting conditions on the mechanical properties of 3mol% yttria stabilized tetragonal zirconia (3Y-TZP), such as biaxial strength, surface elastic modulus, contact hardness and residual stresses induced by sandblasting. The specimens were sandblasted considering two different particle sizes (110, 250µm), two pressures (2 and 4bar) and two impact angles (30° and 90°). Biaxial strength of 3Y-TZP increases when sandblasted with 110µm particles while its decreases with 250µm particles for impact angle of 90°. Strength increases slightly when sandblasting with 30° impact angle regardless of the size of the particle. Elastic modulus and contact hardness were not affected by sandblasting with 110µm particles, and compressive residual stresses are produced down to a depth of ~10µm.

Phase transformation and subsurface damage in 3Y-TZP after sandblasting.

OBJECTIVE: The goal of this work is to investigate t-m phase transformation, and subsurface damage in 3Y-TZP after sandblasting. METHODS: Commercial grade 3Y-TZP powder was conventionally sintered and fully dense specimens were obtained. Specimens were sandblasted using different particle sizes (110 and 250µm) and pressures (2 and 4bar) for 10s. Phase transformation was measured on the surface and in the cross-section using X-ray diffraction and micro Raman spectroscopy, respectively. Subsurface damage was investigated on cross-sections using SEM and in shallow cross-sections machined by focused ion beam. RESULTS: Sandblasting induced monoclinic volume fraction is in the range of 12-15% on the surface. In the cross-section, a non-homogeneous phase transformation gradient is found up to the depth of 12±1µm. The subsurface damage observed was plastic deformation in grains with the presence of martensite plates, and this effect is found to be larger in specimens sandblasted with large particles. SIGNIFICANCE: The extent of subsurface tetragonal-monoclinic transformation and damage induced by sandblasting are reported for different sandblasting conditions. This knowledge is critical in order to understand the effect of sandblasting on mechanical properties of zirconia used to fabricate dental crowns and frameworks.

Tomography of indentation cracks in feldspathic dental porcelain on zirconia.

OBJECTIVES: The objective of this work is to study the crack produced by spherical and sharp indentation on veneering feldspathic dental porcelain in order to understand the morphology of the cracks in the surface and beneath the indentation using a tomographic technique. The geometry of cracks produced under contact loading are directly related to the structural integrity and reliability of dental prosthesis. METHODS: Monotonic Hertzian contact loading and nanoindentation tests were performed on feldspathic porcelain (VITA-VM9) coatings. Residual imprints and the cracks produced by the indentations were characterized by 3-dimensional reconstruction using focused ion beam tomography. RESULTS: Under nanoindentation, the propagating crack deflects due to the interaction with the leucite particles resulting in a crack with a complex morphology. Under spherical contact loading, multiple ring cracks were observed at the surface, with a conical shape beneath the residual imprint. SIGNIFICANCE: These results will help to improve the mechanical performance of these materials by detecting potential causes of failure for the long term structural integrity and reliability of the prosthesis.

Problemas conductuales relacionados con el uso de Internet: Un estudio exploratorio / Behavioral problems related with Internet usage: an exploratory study

El uso problemático de Internet se ha descrito en la literatura psicológica como `adicción a Internet' o `uso patológico de Internet'. Este estudio pretende identificar los posibles efectos relacionados con el uso de la red. Se diseño un cuestionario Web que se difundió a través de la propia red durante 4 semanas. La muestra final estuvo formada por 1664 internautas autoseleccionados de ambos sexos. El análisis de conglomerados (cluster) de las respuestas al cuestionario identificó a un subgrupo de internautas (4,9 por ciento de la muestra) que manifestaban tener problemas frecuentes con el uso de la red, como sentimientos de culpa, deseo intenso de estar o continuar con la conexión a Internet, pérdida de control, pérdida de tiempo de trabajo o clase. También manifestaban una mayor ansiedad y disfunción social, valoradas a través del GHQ-28, que el resto de la muestra. Acceden con frecuencia a servicios de Chat y a páginas de contenido sólo para adultos. Se concluye que el cuestionario utilizado recoge de forma homogénea los problemas relacionados con el uso de la red (=0.91) y una aceptable validez concurrente con los ítems criterio utilizados (r=0.60). Finalmente, se discuten las limitaciones metodológicas de los estudios realizados a través de cuestionarios Web (AU)